A head-mounted display (HMD) is a display device, worn on the head of a user, that has an opaque display optic positioned in front of one (monocular HMD) or each eye (binocular HMD) of the user. Alternatively, an optical HMD is a wearable device, also worn on the head of user that incorporates transparent optics that have the capability of relaying projected images to the viewer while allowing the viewer to see the exterior environment.